MSE 440/540: Processing of Metallic Materials Instructors: Yuntian Zhu Office: 308 RBII Ph: 513-0559 ytzhu@ncsu.edu Lecture 12: Cutting (Shearing) and Bending Text book, Office hour, by appointment Department of Materials Science and Engineering 1

Sheet Metalworking Terminology Punch‑and‑die - tooling to perform cutting, bending, and drawing Stamping press - machine tool that performs most sheet metal operations Stampings - sheet metal products (1) Just before punch; (2) punch pushes into work, causing plastic deformation; (3) punch penetrates into work; and (4) fracture is initiated

Shearing, Blanking, and Punching Three principal operations in pressworking that cut sheet metal: Shearing (cutting) Blanking Punching

Blanking and Punching Blanking (a) - sheet metal cutting to separate piece (called a blank) from surrounding stock Punching (b) - similar to blanking except cut piece is scrap, called a slug

Clearance in Sheet Metal Cutting Distance between punch cutting edge and die cutting edge Typical values range between 4% and 8% of stock thickness If too small, fracture lines pass each other, causing double burnishing (rubbing) and larger force If too large, metal is pinched between cutting edges and produces excessive burr (rough edge) C = clearance http://www.youtube.com/watch?v=VMu7_W0QE3Y

Clearance in Sheet Metal Cutting Recommended clearance is calculated by: c = at where c = clearance; a = allowance; and t = stock thickness Allowance a is determined according to type of metal (4-8%)

Sheet Metal Groups Allowances 1100S and 5052S aluminum alloys, all tempers 0.045 2024ST and 6061ST aluminum alloys; brass, soft cold rolled steel, soft stainless steel 0.060 Cold rolled steel, half hard; stainless steel, half hard and full hard 0.075 Softer metals have smaller allowances

Angular Clearance Purpose: allows slug or blank to drop through die Typical values: 0.25 to 1.5 on each side

Cutting Forces Important for determining press size (tonnage) F = S t L where S = shear strength of metal; t = stock thickness, and L = length of cut edge

Sheet Metal Bending Metal on inside of neutral plane is compressed, while metal on outside of neutral plane is stretched a

Bend Allowance Formula Problem: to determine the length of neutral axis of the part before bending ‘ R where Ab = bend allowance;  = bend angle; R= bend radius; t = stock thickness; and Kba is factor to estimate stretching If R < 2t, Kba = 0.33 If R  2t, Kba = 0.50 (middle line)

V-Bending (1) Before bending (2) After bending Application notes: Low production Performed on a press brake V-dies are simple and inexpensive

Edge Bending (1) Before bending (2) After bending Application notes: High production Pressure pad required Dies are more complicated and costly http://www.youtube.com/watch?v=W6usvOZ9yIw

Springback When bending pressure is removed, elastic energy remains in bent part, causing it to recover partially toward its original shape Quiz: why springback occurs? Plastic + elastic deformation Need overbending to correct

Bending Force Maximum bending force estimated as follows: where F = bending force; TS = tensile strength of sheet metal; w = part width in direction of bend axis; and t = stock thickness. For V-bending, Kbf = 1.33; for edge bending, Kbf = 0.33,

Roll Bending Large metal sheets and plates are formed into curved sections using rolls http://www.youtube.com/watch?v=J09E_luT1pM

HW assignment Reading assignment: Chapters 15,16 Review Questions: 14.3, 14.6, 14.7, 14.8, Problems: 14.1, 14.2, 14.3, Department of Materials Science and Engineering 17